Principles and Possibilities in the Phase Space of Animal Behavior*

We all instinctively recognize behavior: it’s what organisms do, from the motility of single cells to the stunning displays of bird flocks. To understand behavior, however, we must characterize complex, living movement as precisely and completely as its underlying molecular, cellular and network mechanisms. Here, we leverage a low-dimensional but complete representation of the posture of nematode worm C. elegans to reconstruct a continuous 6D phase space of crawling behavior. The reconstruction separates short and long-time dynamics, untangles subtle movement trajectories, and offers a quantitative arena for examining variability and stereotypy. We find that the phase space is organized into 3 conjugate dynamics containg 2 positive Lyapunov exponents which are approximately balanced by dissipative directions. We suggest that a near-Hamiltonian dynamics of coupled, chaotic oscillators underlie the motor control of C. elegans.